Yes, at least with my phone plan. I’m the owner and I COULD read my sisters texts if I really want to. I never have, I don’t see a reason to, but as long as I’m the owner of the Account I can do anything really. I could cut her phone off or block certain features. I have done that before but because that certain thing was annoying her and she didn’t want it.
Sam also answered this question with: "Probably so lol"